Sarcopenia, the loss of muscle mass in older men, has emerged as a major public health problem. Testosterone deficiency has been proposed as one cause for the loss of lean body mass in older. men. However, most older men have testosterone levels that are in the broad normal range, albeit the average testosterone levels in older men are lower than those in younger men. Therefore, it has been speculated that older men might be less sensitive to the anabolic effects of androgens. The purpose of this study is to compare the testosterone dose-response curves in young and older men. Another objective is to examine the testosterone dose-dependency of various androgen-dependent physiologic processes such as protein synthesis, insulin sensitivity, lipids, muscle mass, and sexual function. These data would help select the optimum dose of testosterone for replacement. An additional objective is to compare the testosterone dose-response curves in young and older men in order to determine if older men are relatively insensitive to the anabolic effects of testosterone.